You should be much more in awe of your mobile phone (cellphone) than you are.
In ‘The World’s War’ by David Olusoga, we learn that the first British action of the First World War was “the severing of five underwater telegraph cables that linked Germany to the United States.” The effect of this was that German transatlantic telegrams were routed via Sweden, but Sweden used the British Empire’s network of cables. “That left the German communications susceptible to being intercepted… by the team of cryptologists in the British Admiralty’s Room 40, Britain’s First World War equivalent of Bletchley Park.”
Just to be clear, there were no mobile phones in the First World War. But this passage led me to wonder how many people realise that in modern times, transatlantic mobile phone signals also cross the Atlantic through undersea cables. What I should have done, of course, is randomly sampled a large number of people and surveyed them (including yourself via an online poll, perhaps). Instead, I asked my family. I asked “how do you think your mobile signal gets to the States?” One said “satellites” and one said “radio waves”. Both answers are sensible and wrong. The aim of this story is not to ridicule my family. I’m willing to bet that only a minority of the general public know that undersea cables come into play. Did you know?
That being the case, I thought it would be worth a blog post. If billions of people are attached/addicted to their phones, they might appreciate another reason to be in thrall to them – because how they work really is quite amazing. This post is not about newest versions. It’s not about why the latest iPhone is/isn’t better than the latest Samsung. It’s not about 3G v 4G v 5G. It’s not about apps. It’s about the underlying miracle, the miracle that was as true for a Nokia 3310 as for an iPhone X. How does your phone call hit a moving target so far away?
Networks
To fully appreciate how mobile calls get from place to place, it is useful first to understand the ‘landline’ system. In a world of five people, we could arrange the telephone lines to run from every phone to every other, to make a network, like this.
The disadvantage of this type of network is that connecting every phone directly to every other takes a lot of cable (and expense). It’s bad enough with five phones. Imagine drawing the diagram above with 30 000 yellow circles and joining every one to every other. No, that won’t do, and indeed the phone system never really worked like that. (In the diagram above, ten cables are needed to connect everyone to everyone else; can you work out the number of cables needed for 30 000 people connected in this way? Answer at the end of the post).
Real networks reduced the amount of cable used by introducing a ‘local exchange’, shown in green in the diagram below (only 5 cables needed now, instead of 10). The purpose of the local exchange is to ‘switch’ connections between the phone lines, so that the calls can travel between any pair of phones whenever needed. Now 30 000 phones can be connected easily. Each one does not need to be connected to every other, only to the central node of the network – the local exchange.
When you see old films in which a switchboard operator connects a call manually (often by plugging a jack into a bank of sockets on the switchboard), he/she is working in the green bit… That covers a city, perhaps, but what happens if you want to phone someone in a different city? Different cities were connected by larger cables capable of carrying many phone calls. They were called trunk lines. In the diagram below, the trunk line is the one connecting the two (green) local exchanges.
In old films, you may hear people phoning the operator (in the local exchange) and asking “Operator? I’d like to make a trunk call.” That meant phoning a different city, and asking the operator for permission to use the trunk line.
Over time, as more and more cities and countries became connected, the network became more complex, but it still bears a conceptual resemblance to the previous diagram. Here is a schematic of a modern landline telephone network. It is called the Public Switched Telephone Network (PSTN).
Pick any two phones (yellow circles) on this diagram and trace the simplest route between them. You won’t need to use the international gateway. Depending on the phones you picked, you may or may not need to use the trunk exchanges. Remember this diagram. We will need to come back to it when we make a mobile phone call abroad.
What is a mobile phone signal?
Having learned some things about the PSTN network, we will turn our attention to mobile phone calls.
The first thing to note is that the signal from a mobile phone is an ‘electromagnetic wave’. Its frequency is such that it would typically be labelled as a microwave. The microwaves in your microwave oven are tuned to a frequency that preferentially heats water, and have a power in the order of 1000 watts (W). The microwaves from your mobile phone are not tuned in that way, and have a power in the order of 1 W. The microwave is called a ‘carrier wave’ – the much lower frequency sound information of your voice is encoded onto that carrier wave (in a way that we shall explore in the second part of the blog).
The second thing to note is that two neighbouring mobile phone calls need to use different microwave frequencies. If their frequencies were the same, the two calls would interfere with each other. In fact each mobile phone uses two frequencies per call – one frequency in one direction, and a different one in the other – so that both people can talk at the same time. Walkie-talkies and CB radios use a single frequency for a call, which leads to people saying “over”, to allow the other person to speak (although why “roger” ever became a thing, I’d be interested to find out…).
The cellular network
The mobile network is made of a series of ‘cells’ with a ‘base station’ at the centre of each cell (which is why the US nomenclature ‘cellphone’ makes perfect sense). These base stations are the mobile tower masts you see dotted around cities and countryside. When you use your mobile phone, you are communicating via the base station in the cell where you are. A city/region is split into a large number of cells. The base stations in each cell are connected to the mobile telephone switching office (MTSO – one per service provider). The MTSO handles all the phone connections to the normal land-based phone system (the PSTN), and controls all of the base stations in the region. See the diagram below to help recap that paragraph.
There are reasons for drawing the cells as different-coloured hexagons:
- Hexagons: The hexagons define the area that is closer to that base station than any other. In reality, the cells won’t be perfectly regular hexagons like this, because the base stations won’t be distributed perfectly, but the principle still stands.
- Different colours: remember us saying that two mobile calls at the same frequency will interfere with each other? Well, the signals to and from a base station do not magically stop at the boundary of the hexagon – they die away gradually. So each cell (hexagon) needs to use different frequencies from all its neighbouring cells so that they do not interfere in the region of overlap. The different colours represent different frequencies being used. That also means that each cell (of the seven on a hexagonal grid) is using one-seventh of the available frequencies used by the network.
When you (in the purple cell) make a call to your friend (in the green cell) your signal does not go directly from phone to phone. Instead the following process takes place:
- The call goes via the purple base station to the MTSO.
- The MTSO picks a frequency pair that your phone will use and tells your phone which frequency to use. Your phone is clever enough to roll with that and broadcast on the allotted frequency.
- Once your phone and the base station switch on those frequencies, the call is connected.
- When you talk your voice signal goes from your phone to the purple base station to the MTSO to the green base station to your friend’s phone. When your friend talks the signal traces the opposite route.
- As you move toward the edge of the purple cell, the purple base station notes that your signal strength is getting weaker. Meanwhile, the base station in the cell you are moving toward sees your phone’s signal strength increasing. The two base stations coordinate with each other through the MTSO, and at some point, your phone gets a signal on a control channel telling it to change frequencies, and it switches your phone to the new cell (without you noticing).
Of course, there aren’t just seven cells in a city; the cells repeat in a pattern like this:
A mobile service provider typically gets hundreds of frequencies to use in a city. The genius of a cellular network is the reusability of frequencies in non-adjacent cells. In the diagram above, a cell of any colour can use the same frequencies as the other cells of its colour. Without this system, all phone calls within the diagram would need their own unique frequency, which is not easy to arrange. Also, this system means that the power of a mobile phone can be low – it does not need to transmit across the country, it only needs to transmit across its own cell, and the base station and MTSO will do the rest. This has positive ramifications for battery life, and potentially for human health.
One weird thought is that even if you have a phone call with someone in the same room as you, this kind of process is happening – the signal does not go directly from one phone to the other!
Calls between cities
The description in the previous section applies to calls within a single city or region, when both phone are connected to the same MTSO. What happens if the call is between two phones in different cities, so that two different MTSOs are being used? What connects the two different MTSOs?
The answer is that the MTSOs are connected by the landline system – the PSTN! Your mobile signal travels from your phone to a nearby ‘base station’, to the MTSO, which transfers it into the PSTN. The PSTN then takes it to the MTSO nearest to the target phone, and a mobile signal goes from that MTSO via the nearest base station to the target phone. All of that happens in ‘real time’ as you have your conversation. That situation is shown in the diagram below.
The crazy thing about this is that only the blue part of the diagram is actually using the mobile communication system. The green part of the diagram is where the call has been switched into the (non-mobile-based) PSTN; it is ‘wired’, often by fibre optic cable.
Calls between countries
Finally, we are in a position to understand how transatlantic mobile calls work! As before, the MTSO switches your call into the PSTN, but in this case we need the international gateway. And how does the international gateway get calls across the Atlantic? Via those undersea cables. These undersea cables take ALL the transatlantic phone calls, both mobile and landline-based, from Europe to the States and back. Without them, no transatlantic communications would be practicable (short of high power radios or satellite phones, which people don’t tend to own). This was equally true in the First World War as it is now, hence the actions of the British in severing the German cables. You might reasonably ask exactly how you lay a cable all the way under the Atlantic, and we would say that is a pretty good question – we’ll leave you to look that one up.
There are many undersea cables connecting other countries and continents like this. A good map of the cables can be found at Public Radio International.
It’s not obvious to most of us that mobile calls use the PSTN and international gateway. That’s why, for example, some people (including one member of my family) think that satellites must be involved, reflecting calls directly from one phone to another. You may actually have seen people talking on satellite phones – one example is news broadcasts from journalists in war zones. These are the interviews that have an awkward delay in the conversation, due to the time it takes for the signal to travel to the satellite and back. However, if you are wondering whether you have ever made a phone call involving satellites, then you haven’t. Had you done so, you would have known about it!
So now we know how mobile signals get from one phone to another – microwaves across the mobile network, plus the PSTN, including undersea cables for international calls. But we don’t ‘talk in microwaves’. We talk with sound waves. So how does our phone turn the sound waves of our voices into microwave signals? In other words, we know how the signal gets across the Atlantic, but exactly what is travelling across the Atlantic? Well, we will describe that in the second part of this post…
(Answer: about 450 million cables needed for 30 000 phones, all directly connected to every other)